The present invention relates to a semiconductor device composed of a group III–V nitride semiconductor represented by a general formula (InxAl1−x)yGa1−YN (where 0≦X≦1 and 0≦Y1 are satisfied) and to a method for fabricating the same.
A group III–V nitride semiconductor such as GaN, AlGaN, InGaN, or InAlGaN, i.e., a so-called gallium nitride-based semiconductor is an important semiconductor for an optical device having a short oscillating wavelength such as a semiconductor laser device outputting, e.g., a blue laser beam. The applications of the gallium nitride-based semiconductor are not limited to the optical device. In recent years, attention has been focused on the gallium nitride-based semiconductor for its high dielectric breakdown field strength, high thermal conductivity, and high electron saturation velocity so that gallium nitride-based semiconductor is considered to be promising also as the material of an RF power device.
In an AlGaN/GaN heterojunction structure composed of aluminum gallium nitride (AlGaN) and gallium nitride (GaN) which are stacked in layers, in particular, electrons are accumulated at a high density in the vicinity of the heterojunction interface between AlGaN and GaN to form a so-called two-dimensional electron gas. The two-dimensional electron gas exhibits a high mobility since it exists spatially separated from a donor impurity used to dope AlGaN. Therefore, the AlGaN/GaN heterojunction structure achieves the effect of reducing a source resistance component when used in a field effect transistor (FET).
Since the distance d from a gate electrode to the two-dimensional gas is normally as small as several tens of nanometers, a ratio Lg/d between a gate length Lg and the distance d, which is termed an aspect ratio, can be held at a large value of 5 to 10 even if the gate length Lg is as small as about 100 nm. Hence, the use of the AlGaN/GaN heterojunction structure offers an advantage of easy fabrication of a FET with a reduced short channel effect and an excellent saturation characteristic.
The electron velocity of a two-dimensional electron in a high-field region of about 1×105 V/cm in the AlGaN/GaN-based heterojunction structure is double or more the electron velocity thereof in a gallium arsenide-based (GaAs-based) FET which is currently prevalent as an RF transistor, i.e., an AlGaAs/InGaAs heterostructure FET. In addition, the density of electrons accumulated at the heterointerface becomes as high as 1×1013/cm2 when the composition of Al in AlGaN is 0.2 to 0.3, which is about three to five times as high as the density of electrons in the GaAs-based device.
Since the dielectric breakdown field strength of the GaN-based heterostructure FET is about ten times as high as that of the GaAs-based FET, the application of a drain voltage therein which is ideally about ten times a drain voltage applied in the GaAs-based FET becomes possible on the assumption that the GaN-based heterostructure FET and the GaAs-based FET have the same device patterns. This renders the GaN-based heterostructure FET promising as an RF power device capable of generating an output power which is at least 5 times, ideally about 10 times as high as or higher than an output power generated by the GaAs-based power device but it also has numerous problems to be solved.
One of the problems associated with the GaN-based heterostructure FET is a large surface leakage current between the gate and drain thereof. A gate electrode composing a GaN-based heterostructure FET is a so-called Schottky gate electrode normally composed of a metal material with a relatively large work function coated directly on a semiconductor. As a metal material composing the Schottky gate electrode, a material having a large work function such as nickel (Ni), palladium (Pd), or platinum (Pt) is used appropriately.
If a current-voltage characteristic between the gate and drain is examined after the metal material is vapor-deposited, an abnormally large leakage current is observed frequently at a reverse voltage and a leakage current value cannot be reduced consistently. The large leakage current significantly increases an idle current component to the power device when a high negative voltage is applied to the gate electrode. As a result, the advantage of the GaN-based heterostructure FET that it can be driven at a high drain voltage cannot be used effectively any more, which presents a critical problem.
The occurrence of such a large gate leakage current may be attributed to a reaction between an oxide film (natural oxide film) formed on a surface of the GaN-based semiconductor and the metal material coated thereon and to a reaction between the surface of the GaN-based semiconductor and the metal material.
To prevent such reactions and thereby reduce the gate leakage current, there has been proposed a so-called MIS (Metal-Insulator-Semiconductor) structure or a MOS (Metal-Oxide-Semiconductor) structure in which an insulating film composed of a silicon nitride (SiN), a silicon dioxide (SiO2), or the like is deposited on the surface of the GaN-based semiconductor and a gate electrode is formed on the deposited insulating film.
However, since the oxide film mentioned above exists at the interface between the gate insulating film and the GaN-based semiconductor or a trap is easily introduced into a surface of the semiconductor by a surface treatment performed in the fabrication process, a GaN-based FET using the MIS structure or MOS structure described above is not necessarily stable because of the current-voltage characteristic thereof which varies depending on the operating frequency.